Multiwavelength lasers based on Quantum Dots (QDs) are known in the art. They produce light that, in the frequency domain, consists of a few to hundreds of lasing modes that are discretely and substantially uniformly spaced apart in a band (also known as a “colour”).
According to the prior art, (single-band) multiwavelength lasers have been produced with different gain materials such as rare-earth-doped fibers, bulk or quantum-well (QW) semiconductor waveguides, and by using different techniques such as active overlapping linear cavities [5], a high birefringence fiber loop mirror [6], intracavity polarization hole burning [7], distributed Bragg grating [8], an elliptical fiber [9], intracavity tunable cascaded long-period fiber gratings [10], a sampled chirp fiber Bragg grating [11], a self-seeded Fabry-Pérot laser diode [12], spatial mode beating within a multimode fiber section [13], multi-cavity oscillation [14], and others [15-18].
Because of the nature of large homogeneous broadening of gain media, the resulting multiwavelength lasers are sensitive to variations in intracavity gain and/or loss. Because every lasing mode shares mostly the same population inversion reservoir, all lasing modes compete continuously with each other for a larger share of this reservoir. Given the unavoidable fluctuations in electrical and optical fields within the optically active medium, the intracavity gain-loss balance for any lasing mode could be broken, resulting in fluctuations of the laser output. Consequently, the number of lasing modes in one band is very limited and the intensity of each lasing mode fluctuates.
To overcome these problems, a new gain material, semiconductor quantum dots (QDs), were introduced for generating multiwavelength lasers. The nature of QDs as active gain material permits inhomogeneous gain broadening to suppress the competition among lasing modes, leading to single-band QD-based multiwavelength lasers with tens or hundreds of lasing modes [2-3, 19], which have been demonstrated with high intensity stability and high signal-to-noise ratio.
So far, QD-based mode-locked lasers at different wavelengths and various repetition rates have been successfully demonstrated [24-26] owing to the inhomogeneous spectral broadening based on the statistical distribution in QD sizes and shapes as well as the subpicosecond gain recovery times.
Because of their compact size, mechanical stability, low power consumption, direct electrical pumping, easy operation, and manufacturability, (single-color) mode-locked lasers are promising as cost-effective and versatile light sources for many applications such as: all-optical clock recovery and high bit rate transmission in optical communications [20], coherent manipulations of qubits in quantum computation [21], generation of microwave or THz radiation in spectroscopy [22], ultrafast optical processing, multi-photon imaging, and laser machining [23]. These and other applications are possible for multi-band mode-locked lasers.
Multi-band (or multi-colour) lasers, lasers that emit at multiple bands are also known. Mode-locked lasers operating simultaneously at two or more bands have been developed in the visible wavelength range with the use of two cavities sharing a single Ti: sapphire crystal [27], or by Raman scattering [28-29]. However, as far as Applicant knows, no work specially addressing two- or multi-band mode-locked lasers using QD active media has been reported.
Recently two-band QD-based multiwavelength continuous wave (CW) lasing from both a ground state and an excited state has been reported [4] near the 1.3 μm wavelength neighbourhood. Because the ground state and excited state have fixed energy-level structures, the band positions in this two-band laser are also fixed. This provides no flexibility in the positions and distributions of the bands.
There is a need for multi-band multiwavelength lasers from QD materials with some flexibility regarding the positions and distributions of the channels within the bands. Furthermore it would be desirable to produce intraband, and/or multiwavelength mode-locked lasers.